The present invention relates to a vacuum interrupter, particularly to a vacuum interrupter including a vacuum envelope a major portion of which consists of a generally cylindrical (including bottom-cylindrical) metallic housing and a disc-shaped insulating end plate, made of insulating ceramics, hermetically brazed to an opening end of the metallic housing.
For the purpose of size-down and production cost-down, a vacuum envelope of a vacuum interrupter which, as described above, includes in the main a metallic housing and insulating end plate has replaced one which includes in the main an insulating cylinder made of insulating ceramics or crystallized glass and a metallic end plate hermetically sealed to the opposite opening ends of the insulating cylinder (See U.S. patent application No. 276,862).
Since the insulating surface distance of an insulating end plate is shorter than that of an insulating cylinder if vacuum interrupters of the two types described above are equal in interruption capacity, dielectric strength per unit length of the insulating end plate must be greater than that of the insulating cylinder. Therefore, voltage distribution per unit length for the insulating end plate, i.e., electric field intensity therein, is necessarily greater than that in the insulating cylinder.
Moreover, in case of a vacuum envelope as described above including in the main a metallic housing and an insulating end plate, both the members are conventionally united with each other by hermetic-brazing the surface of an opening edge of the metallic housing in the surface-to-surface manner to an extremely thin metallized layer which covers the outer peripheral region of the sealing surface of the insulating end plate. In view of the shape, the metallized layer, the inner edge region of the surface and the extremely narrow side surface thereof caused to appear within the vacuum envelope, has a capability of electric field concentration.
Moreover, where the insulating end plate centrally has an aperture through which an electrode lead rod extends, and where metallized layers cover the inner peripheral region as well as the outer peripheral region in the sealing surface of the insulating end plate, the electric field concentration which is effected at the respective extremely thin edge surfaces of both the metallized layers, opposed to each other, is ready to trigger insulation breakdown between the metallized layers.
Moreover, since the metallized layer microscopically has small protrusions thinly spread over the surface, which causes electric field concentration, the layer is capable of triggering the internal insulation breakdown of the vacuum envelope.
Moreover, the metallized layer is made of Mn, W, Mo or the like which has a pronounced tendency to effect electron emission. This feature also leads to the internal insulation breakdown of the vacuum envelope.